1. Field of the Invention
The present invention relates to a traction control system for an automotive vehicle and, more particularly, to a traction control system for controlling torque to driven wheels so as not for wheel slip to exceed a predetermined value.
2. Description of the Related Art
If the driven wheel of a vehicle slips at a relatively large amount relative to a surface of road, for instance, when an accelerator pedal is depressed to accelerate the vehicle, the driven wheel cannot offer appropriate grip so that suitable running characteristics cannot be gained. In order to suppress the wheel slip from exceeding a predetermined degree, the automotive vehicle is provided with a traction control system that can supply braking force to the driven wheels or that can decrease engine power.
Such a traction control system usually performs the traction control by setting a target slip value in accordance with a friction coefficient .mu. on a surface of road. Such a traction control system is adapted to perform operations to decrease torque to the driven wheels by supplying the braking force to the driven wheels or by decreasing the engine power, when the slip value of the driven wheel exceeds a predetermined value. In this case, the road surface friction coefficient .mu. to be employed for setting the target slip value may be estimated on the basis of a peripheral speed of a non-driven wheel or on the basis of the peripheral speed of the non-driven wheel and peripheral acceleration of the non-driven wheel, as described in Japanese Patent Laid-open Publication (kokai) No. 60-99,757.
Another example of a traction control system is proposed, for example, by Japanese Patent Laid-open Publication (kokai) No. 63-166,649, which is so adapted as to implement the traction control by operating a braking system for braking driven wheels, together with decreasing the engine power, only when the slip of the driven wheel is of a relatively large scale. This traction control system, however, performs the traction control by decreasing the engine power without using the braking system for the driven wheels, when the wheel slip of the driven wheel is of a relatively small scale, with durability of the braking system taken into account. This traction control system is arranged to adjust an amount of intake air to be supplied to a combustion chamber of an internal combustion engine through its intake passage when the operations for decreasing the torque for driving the wheels are carried out by decreasing the engine power when the slip of the driven wheel becomes larger than a predetermined value. The amount of intake air is adjusted by adjusting an opening angle of an auxiliary throttle valve disposed in the intake passage of the internal combustion engine separately from a main throttle valve disposed so as to open an opening angle thereof in association with an accelerator pedal. The auxiliary throttle valve is driven by an actuator disposed so as to be controlled by a control signal for changing the wheel slip of the driven wheel to a target slip value when the wheel slip thereof became larger than the predetermined slip value.
When the slip of the driven wheel is suppressed, for instance, due to the state of the surface of road and the slip value thereof becomes a relatively smaller slip value, thereby allowing a peripheral velocity of the driven wheel to approximate to a peripheral velocity of the non-driven wheel, the slip of the driven wheel relative to the surface of road may not be converted into the predetermined target slip value at which an increase in the driving force can be transmitted to the surface of road with high efficiency even if the accelerator pedal have been depressed. In this case, the problem may arise that feeling of good acceleration cannot be attained when the accelerator pedal is depressed with the attempt to gain good acceleration.